1. Technical Field
This disclosure relates to delay locked loops (DLLs), and more particularly to a DLL training protocol during a change in the reference clock frequency.
2. Description of the Related Art
Many types of devices use delay locked loops (DLLs). Typically, a DLL is used to establish and maintain a particular phase relationship with a reference clock or other signal and to provide one or more delayed versions of that reference signal. When a DLL is first powered up, the DLL may enter a training mode to acquire and lock onto a reference signal edge. In addition, in many DLLs the delay line may be set up to provide the required amount of delay and thus a phase offset for the intended application. In some DLLs, the delay line includes a number of delay elements, each providing a particular amount of delay.
Generally, the number of delay elements will not change as long as the reference signal frequency remains the same. However, in situations where the reference clock frequency changes, conventional DLLs will typically have to execute a retraining to lock and to reconfigure/recalculate the number of delay elements to provide the required phase delay. This process can take time. Depending upon the specific application, the retraining time may be unacceptable.